Implement multi-level page tables.

Implement multi-level page tables.

Define L1_MAP_ADDR_SPACE_BITS to be either the virtual address size
(in user mode) or physical address size (in system mode), and use
that to size l1_map.  This rewrites page_find_alloc, page_flush_tb,
and walk_memory_regions.

Use TARGET_PHYS_ADDR_SPACE_BITS for the physical memory map based
off of l1_phys_map.  This rewrites page_phys_find_alloc and
phys_page_for_each.

Signed-off-by: Richard Henderson <rth@twiddle.net>